In image forming apparatuses of the electro-photographic method such as copying machines, printers, facsimiles, and multifunction peripherals provided with all these functions, a latent image corresponding to the original is formed on a photoreceptor drum, this latent image is made visible by giving toners to it, this visible toner image is transferred onto a recording material, and after that, the toner image on the recording material is fixed and then the recording material is discharged to outside the apparatus.
Further, in the case of forming color images, latent images of colors Y, M, C, and K corresponding to the colors of the original are formed on four photoreceptor drums, the toner images of four colors made visible are primary-transferred on to an endless intermediate belt, then they are secondary-transferred on to a recording medium, then toner image transferred on to the recording medium is fixed, and then the recording material is discharged to outside the apparatus.
As a fixing device for fixing such toner images is present a fixing device of the heated roller method that applies heat and pressure by gripping and conveying the recording medium with the toner image transferred on to it in a nip portion formed by a fixing roller having inside it a heating member such as a halogen lamp, etc., and a pressure roller that applies pressure on the fixing roller, and such a fixing device is being used widely because its construction is simple.
However, in such a fixing device, in order to achieve high speed it is necessary to supply sufficient amount of heat to the toner and the recording medium, and to do this it is necessary to increase the nip width. In order to increase the nip width, it is possible to consider increasing the load with which the pressure roller presses the fixing roller, the thickness of the elastic layer formed from silicone rubber, etc., inside the pressure roller, or the diameters of the two rollers, etc.
However, if the load or the thickness of the elastic layer of the pressure roller is increased, the nip width along the axial direction may sometimes become uneven, and there is the possibility of uneven fixing or wrinkles of the recording material occurring. Further, if the roller diameters are increased, there is the problem that not only the fixing device becomes big but also the warming up time becomes long.
In order to solve this problem, a fixing device of the so-called FBNF method has been disclosed in the Unexamined Japanese Patent Application Publications (see publication No. 2005-173441, No. 2004-198655, and No. 2004-206105), wherein the fixing device is provided with a fixing roller having an elastic layer made of silicone rubber and a heating member such as a halogen lamp, etc., built in at the center and rotating, an endless fixing belt that rotates being driven by the fixing roller, a pressure pad that presses against the fixing roller via the fixing belt from the inside of the fixing belt, and a low friction sliding sheet placed between the fixing belt and the pressure pad, and in the nip portion formed by the fixing roller and the fixing belt, the recording material on to which the toner image has been transferred is heated and pressed while being gripped and conveyed.
According to this fixing device, the fixing belt pressed to the fixing roller by the pressure pad deforms elastically, and a wide nip section is formed between the fixing roller and the fixing belt. Consequently, it is possible to achieve high speeds, and also the fixing device does not become big. In addition, since the thermal capacity of the fixing belt is small, the warming up time becomes short and it is possible to save energy.
Further, although in Unexamined Japanese Patent Application Publication No. 2005-173441 the sliding resistance is being reduced by supplying lubricating agent between the fixing belt and pressure pad in a fixing device of the FBNF method, since the lubricating agent is not retained in a stable manner, a surface roughness of Ra=0.3 to 1.6 μm is formed on the inner peripheral surface of the fixing belt thereby increasing the lubricating agent retention capacity. Further, polytetrafluoroethylene (hereinafter called PTFE) has been disclosed as the material of the releasing layer coated on the surface of the fixing belt, and PTFE has also been disclosed as the material of the sliding sheet.
In Unexamined Japanese Patent Application Publication No. 2004-198655, similar to Unexamined Japanese Patent Application Publication No. 2005-173441, in order to increase the lubricating agent retention capacity, the surface contacting with the fixing belt in the sliding sheet (pad cap) has been made a rough surface. It is desirable that the surface roughness of the undulating surface is Ra=0.5 to 10 μm. Further, PFA has been disclosed as the material of the releasing layer of the fixing belt, and PES (polyester sulfone), etc., have been disclosed as the material of the sliding sheet.
In Unexamined Japanese Patent Application Publication No. 2004-206105, although the conventional sliding sheet is formed to be porous in order to retain the lubricating agent, since it cannot withstand use for a long time, the sliding surface of the sliding sheet is made porous and to include a fluoroplastic. In addition, even PTFE has been disclosed as the material of the releasing layer of the fixing belt and also PTFE has been disclosed as the material of the sliding sheet.
As has been disclosed in Unexamined Japanese Patent Application Publication No. 2005-173441, Unexamined Japanese Patent Application Publication No. 2004-198655, and in Unexamined Japanese Patent Application Publication No. 2004-206105, in a fixing device of the FBNF method, because of placing a sliding sheet between the fixing belt and the pressure pad, the sliding resistance of the rotating fixing belt has been reduced since the fixing belt and the pressure pad do not come into direct contact with each other. In addition, reducing the sliding resistance is also made by supplying a lubricating agent between the fixing belt and the sliding sheet.
However, since the fixing belt is in sliding contact with the sliding sheet, there is a certain amount of sliding resistance and at least some abrasion is caused in both of them. In addition, although it is necessary to retain the lubricating agent definitely in the nip portion, conventionally it was not sufficient to prevent the abrasion and an efficiency to retain the lubricant was deteriorated through a long time usage.
An object of the present invention is to solve the above mentioned problem.